1) Field of the Invention
This invention relates to structure and a method for manufacturing semiconductor devices and particularly to a photolithographic method of producing semiconductor devices and more particularly to a structure and a method for applying a primer to a wafer before a photoresist layer is applied.
2) Description of the Prior Art
In standard semiconductor manufacturing, a substrate surface is "primed" before the substrate is coated with photoresist. The primer or priming is a pre-resist coating of some material that will increase the adhesion of the resist to the substrate surface. The primer reduces the mount of lateral etching or undercutting. The adhesion of photoresist to wafer and mask surfaces is a function of the amount of inter-molecular bonding between the resist and the surface onto which it is applied. In addition to this fundamental adhesive force, which is chemical in nature (molecular and atomic bombing), there are "external" forces and factors that influence the ability of resist to withstand lateral etching. These factors include: the moisture content of the wafer and mask surfaces, the wetting characteristics of the resist on surfaces, the type of primer used and its method of application, the resist chemistry, the contamination or surface defects that occur at the interface of the resist and wafer surfaces.
Adhesion of photoresist to the substrate surface is reduced when the photoresist is applied over a hydrated substrate surface. The presence of water on the oxide surface allows the wet etchants to penetrate easily between the resist and the wafer or mask surface. The penetrating etchants etch the oxides under the resist making the oxide pattern smaller thus causing an "undercutting" problem. The smaller, less accurate oxide patterns can cause device failures and change the parameters of the subsequent devices. To remove moisture from the substrate/photoresist interface, a primer is applied to the substrate surface before a photoresist layer is formed thereover. The primer reacts with the moisture to "tie up" the moisture so that photoresist adhesion is increased. For example, primers such as hexamethyldisilizane (HMDS) are used to react with substrate-oxide surfaces so as to tie up molecular water. However, an accumulation (or liquid droplets) of primer on the substrate surface before the resist is applied will also affect the photoresist coverage and cause defects in the resist pattern which is subsequently formed.
Many solutions have been used to promote the adhesion of resists to wafer and other semiconductor surfaces. Adhesion promoters such as hexamethyldisilizane (HMDS), Trichlorophenylsilane (TCPS), bistrimethylsilylacetimide (BSA), and Xylene are used mainly on silicon dioxide surfaces, but can be used on other oxide, metal, and glass surfaces. HMDS is by far the most commonly used adhesion promoter and is preferred for a number of reasons. The adhesion promoters containing chloride are undesirable because of corrosion problems. Also, the toxicity and overall bonding effectiveness are less compared with HMDS. Some postbaking is nearly always required after treatment with all but HMDS, and the length of time over which an adhesive promoter remains effective on a wafer is much longer with HMDS compared to the others.
The application of adhesion promoters to wafer surfaces can be performed by one of three coating techniques: (1) wafer spin coating, (2) dip coating, or (3) vapor coating. Vapor priming is the use of primer vapors to provide conversion of the silicon dioxide or other surface. Vapor priming has an advantage of reducing contamination by solid particles since only vapors touch the wafer. A typical vapor priming system consists of a primer source, a wafer priming chamber and piping connecting the bubbler to the chamber. In operation, HMDS vapor and nitrogen flow form the source through the piping to the wafer priming chamber where the vapor flows past the wafers thus coating the wafers with an even layer of HMDS primer.
A problem with vapor priming of wafers is that often the primer condenses on the wafer thus forming liquid drops 27 of primer on the wafer surface. See FIG. 1. In some situations this primer condensation 27 is caused by primer condensation in the piping leading to the wafer chamber or from condensation on the wafer priming chamber walls.
This primer condensation on wafers will cause severe defects in wafers. In particular, the condensation will cause photoresist residue on the wafer surface after the photo resist is developed and etched. The defective photoresist pattern causes inadequate structures to be formed on the wafer. This vapor primer condensation occurs in the piping as well as in the priming chamber. This primer condensation problem becomes worse as the primer chamber is idle for longer periods of time, such as down times for fixing the machine, preventive maintenance, or line shut downs.